Pneumatic vehicle tires are generally reinforced with steel cords having substantially round cross-sections. A plurality of single filaments or strands are twisted together to form such a cord. Flat wires for the reinforcement of pneumatic tires have an improved lateral stiffness, require a thinner rubber ply, and have a better compression performance and a greater elongation at break than cords having substantially round cross-sections. However the latter have a better tensile strength and better fatigue performance than flat wire.
EP 0264145A1 discloses that a steel cord comprises a layer of steel filaments that are wound around an untwisted monofilament which has an elongated cross-section and a width-to-thickness ratio of at least 2. But in this cord structure that the filaments in the layer line contacts with the adjoining filaments, the space between each pair of sheath filaments is so narrow that rubber can not easily penetrate. Besides, as the width-to-thickness ratio becomes larger, the productivity is lowered.
EP 0551124A2 discloses that a steel cord comprises one wave-form or helical core steel filament, and a plurality of sheath steel filaments disposed around the core steel filament, which allows better rubber penetration for improved resistance to corrosion. However, such construction increases the thickness of the ply. In addition, the proposed structure has the same stiffness in the plane and perpendicular to the plane of the core, so it is not so flexible to deformation perpendicular to the plane of the core, which is disadvantageous for its use in belt or breaker plies of pneumatic radial tires.